New Zealand has not signed any free trade agreement that will stop plans to sell cigarettes in plain packaging, Prime Minister John Key says.

That's despite threats by a consortium of American organisations who issued a statement over the weekend claiming tobacco firms' logos and branding were protected by laws and international treaties.

The US Chamber of Commerce and several other organisations said the tobacco industry had "legitimate trademark protection and branding".

Associate Health Minister Tariana Turia last week announced a ban on branded packets as part of a bid to get New Zealand smokefree by 2025.

The move was aimed at preventing young people getting hooked on smoking rather than deterring existing smokers.

Key said this morning that New Zealand had not signed up to any free trade agreement that would leave the country open to a legal challenge over plain packaging of cigarettes.

"We retain sovereign rights for lots of things and if we want to have the sovereign right that says 'if you want to bring cigarettes into New Zealand they have to be plain packaging' ... I haven't seen anything yet that tells me our law would prevent us from doing that."

New Zealand was yet to sign up for the Trans Pacific Partnership agreement.

People could test their rights in court but that did not mean they were correct, Key told TVNZ's Breakfast show.

Moves by the Australian Government to introduce plain packaging from December have led to a battle in the High Court.

The American statement was also signed by the TransAtlantic Business Dialogue, the Emergency Committee for American Trade, the National Association of Manufacturers, the United States Council for International Business and the National Foreign Trade Council.

The Government here will introduce legislation once a public consultation exercise is complete.